1. Field of the Invention
This invention relates generally to the construction of wells for the production of petroleum products and more specifically to the construction and completion of multilateral branches from a main well bore to enable the production of petroleum products from several subsurface zones. Even more specifically the present invention concerns methods and apparatus for connecting a lateral branch liner to a main well bore to achieve predictable and stable mechanical connectivity at the lateral junctions of branch well bores to the main well bore to counter the problems of formation instability at the branch junction which may cause partial or total obstruction of the lateral or main bore at the level of the lateral junction.
2. Description of the Related Art
In the field of multilateral well construction and production operations one of the most valuable attributes of a junction is the connectivity of lateral branches with the main bore. Partial or total loss of connectivity of the main bore with a lateral branch may cause fluid production loss. Major connectivity problems may also result in partial or total obstruction of the main or lateral bore at the level of the lateral junction. The consequences of such problems are a substantial penalty to the operator of a well in the form of lost opportunity, increased operating cost, or lost production. The root cause of not being able to achieve or maintain connectivity at a lateral junction can be divided into two general areas: mechanical integrity problems and production of solids from the formations surrounding the junction. Mechanical integrity problems are usually a combination of design factors limiting the strength of, and mechanical forces applied by the surrounding formations onto, the connecting equipment. Production of solids from surrounding formations occurs when the junction technique does not achieve a consistent connectivity by means of mechanical liner tie-back solutions. This can be the case when a liner is connected to the parent well bore by means of cement or any similar joining technique which does not withstand tensile or shear forces that may be induced by formation pressures or subsidence or any other formation movement at the level of the lateral junction.
One form of prior art is the use of a mechanical connection embedded with a casing section which has one or a plurality of pre-fabricated windows. Although such solution provides a possible connection of the lateral liner to the parent well bore, it requires a special vessel to be installed in line with the casing string at a specific depth and, more importantly, with a correct orientation with respect to earth gravity in order to place the pre-fabricated window in the direction of the projected lateral branch. This method requires very thorough well planning and delicate control of parent casing running procedures. Another drawback of this method is that connective template and retaining features are run with the parent casing and must therefore remain protected from any mechanical abuse while drilling in the parent section or drilling the lateral branch. Such method and apparatus generally requires other additional equipment to complete the well with lateral re-entry capability. Such device may be or similar to equipment for through-tubing re-entry by means of a secondary template. As a result a junction fully completed with such method will generally offer limited diameter to access the lower section of a parent well.
Another commercially available form of lateral connectivity does not require pre-orientation of the parent casing since it is implemented by milling lateral windows in installed well casing. The lateral liner is retained into the parent well bore and cemented into place. A window is then milled into the lateral liner in order to re-establish communication between the lower section of the main bore and the lateral and upper section of the main bore. However, most mechanical integrity of the lateral connection involves cement or similar filling material placed in the space surrounding the junction. As explained above, the cement lacks sufficient structural integrity, especially when shale in the formation shifts from time to time as the formation changes consistency due to production of fluid therefrom or due to production fluid from a lower or different formation, so that the cement becomes fractured and impairs the connectivity of the branch junction.
Another form of lateral connectivity is accomplished by conveying a liner into the lateral branch after milling a window in the parent casing and after lateral drilling. The liner is cemented into place while the liner is held in the parent well with a liner hanger. After the cement has set, cement excess and the liner top is "washed-over" with adequate milling and fishing tools. A deflection tool left in the parent well is then retrieved and this should normally leave a full bore in parent well. Completion equipment is then set in the junction, assuming an indexing packer is left below the junction. The major drawbacks of such method are similar to those described above, since the mechanical integrity of the junction involves cement or similar filling material which has been placed while setting the lateral liner.
Another form of lateral connectivity takes the form of a prefabricated outlet which fits mechanically within a special vessel that is connected in line with the parent casing. The special vessel supports a selective positioning profile and an orienting profile. The outlet is conveyed with the parent casing in retracted position and deployed in the main bore by action of an expansion tool which extends the outlet around a hinge placed on top of the outlet. The outlet and vessel are interlocked and sealed after the outlet is fully extended. A liner can be set, and retained in the lateral outlet bore by means of a liner-hanger-packer device. Such method requires a very complex deployment process and more importantly requires the special vessel to be placed and oriented in a precisely predetermined position while running the parent casing, and requires the outlet to be extended before cementing. Also the fact that a lateral outlet is pre-installed in the junction restricts the size of lateral drilling with conventional methods.